Fiber optic cables include optical fibers that transmit signals, for example, voice, video, and/or data information. Optical fibers require cabling to protect the relatively fragile silica-based optical fibers and to preserve the optical performance thereof. For example, because optical fibers are not ductile they must be protected from external forces such as tensile forces. Additionally, optical fibers require protection from macro-bending and/or micro-bending to inhibit undesired optical degradation.
In order to meet these requirements, fiber optic cables designed for indoor, outdoor, or indoor/outdoor applications typically have a cable core surrounded by a sheath system that generally includes a cable jacket. For example, a cable core may include an optical fiber, a strength element, and/or a separation layer. The separation layer generally is on the outer surface of the cable core and prevents the extruded cable jacket from sticking to the cable core and/or optical fiber, thereby allowing relative movement between cable jacket and the cable core and/or optical fiber. The relative movement therebetween, for example, during bending and/or flexing of the fiber optic cable inhibits stress and/or strain on the optical fiber, thereby preserving optical performance. Additionally, the cable jacket protects the optical fibers from, for example, environmental effects.
The strength element of a fiber optic cable is intended to carry tensile loads applied to the fiber optic cable inhibiting, for example, tensile stress and/or strain from being applied to the optical fibers within the cable. Different types of strength members may be used in fiber optic cables, for example, metal wires, glass-reinforced plastics, and/or aramid fibers. Fiber optic cables may employ a single type of strength member or combinations of different types of strength members. However, different types of strength members may have different characteristics, for example, glass-reinforced plastic rods and/or metal wires additionally provide an anti-buckling characteristic to the fiber optic cable. However, strength members having anti-buckling characteristics generally increase the stiffness of the fiber optic cable, thereby increasing the bending radius of the fiber optic cable. Thus, fiber optic cables having strength members with anti-buckling characteristics are generally unsuitable for small bend radius applications, for example, splice trays and/or as an interconnect cable assembly.
Fiber optic cables having relatively flexible strength members, instead of stiff strength members, such as aramid fibers are generally more flexible and are suited for, among other applications, interconnect cable assemblies and/or within splice trays. Moreover, relatively flexible strength members may also, among other functions, provide a separation layer between the cable core and the cable jacket. However, fiber optic cables without anti-bucking members are generally susceptible to optical performance degradation due to shrinkage of the cable jacket during manufacture and/or due to ambient environmental changes in the field such as temperature and/or humidity.
For example, an interconnect cable assembly may include a fiber optic cable having a cable jacket extruded over a cable core with aramid fibers generally surrounding an optical fiber. The aramid fibers act as both a strength element and a separation layer. However, the aramid fibers do not provide anti-buckling. Consequently, the interconnect cable assembly is susceptible to optical performance degradation due to the shrinkage of the cable jacket because as the cable jacket shrinks the aramid fibers do not inhibit the optical fibers from becoming undulated and/or buckled.
The formation of the cable jacket of a fiber optic cable of the interconnect cable assembly is accomplished through an extrusion process where the jacketing material is melted at a relatively high temperature and extruded over the cable core that passes through, for example, a cross-head extruder. After the jacketing material is extruded over the cable core, the fiber optic cable passes through a water trough to quench the relatively hot cable jacket. When the jacketing material, for example, a polyvinyl chloride cools during the quenching process shrinkage of the cable jacket can occur. This shrinkage of the cable jacket can result in an undulated cable core causing undesirable compressive axial stress and/or strains being applied to the cable core and/or optical fiber, which can cause undesirable optical attenuation.
Additionally, there are other sources of cable jacket shrinkage that may cause undesired optical degradation. For example, in the field an interconnect cable assembly can also experience relatively large environmental temperature and/or humidity variations. Such variations can result in, for example, cable jacket expansion and contraction. The expansion and contraction of the cable jacket can cause tensile and compressive forces to be transferred to the optical fibers within the interconnect cable assembly. For example, shrinkage of the cable jacket can cause undulation and/or buckling of the optical fiber(s), thereby resulting in undesired optical degradation in the interconnect cable assembly.
The optical performance of a fiber optic cable can be measured, for example, by measuring an insertion loss through a fiber optic interconnect cable assembly. Insertion loss is a measure of a fraction of the signal light that is lost in the interconnect cable assembly and is, generally, measured in decibels. In general, insertion loss is an undesired result because it results in a weaker optical signal. Additionally, light can be lost if the end faces of the fibers are separated; therefore, the end faces of the fibers should also be maintained in virtual contact by the optical connectors. Fiber-to-fiber separation also implies an insertion loss due to Fresnel reflections at one of the two glass end interfaces.
There are different methods to reduce shrinking of the cable jacket to preserve optical performance. For example, one method to reduce shrinking of the cable jacket is to embed a strength element having an anti-buckling characteristic within the cable jacket. This may generally inhibit the cable jacket from shrinking during the manufacturing process when the cable jacket is cooling after being extruded. However, this results in a relatively stiff cable generally unsuitable for use in small bend radius applications and/or as an interconnect cable assembly.
Other methods to reduce the shrinkage of a cable jacket that do not require embedding strength elements within the cable jacket are known. For example, disclosed in U.S. Pat. No. 6,324,324, is a cable jacket that requires a resin containing a filler material with a high aspect ratio and possibly a coupling agent to aid adhesion between the filler material and the resin. The volumes of the filler material ranges between 1.5% and 25%. However, this method requires mixing the different materials, which may result in a non-homogeneous mixture. Additionally, mixing the materials of the cable jacket adds another level of complexity to the manufacturing process and can result in increased manufacturing costs.